Infiltration of the Plant Surface by Aqueous Cell Suspensions

Water or aqueous cell suspensions of bacteria may infiltrate apertures as well as wounds on fresh fruits and vegetables during harvest and handling [30,34, 51-53]. This infiltration can directly internalize microbes and can be either spontaneous or pressure driven. Schonherr and Bukovac [22] observed spontaneous penetration of stomata on leaves by water that was amended with a surfactant. A biosurfactant produced by Pseudomonas fluoresceins alters the wax crystals on the surface of broccoli florets and may aid in spontaneous penetration of that structure by plant pathogenic strains of this bacterium [3]. Fresh wounds also appear susceptible to spontaneous infiltration by surface moisture [43,50].

Pressure-driven infiltration of fruits and vegetables means that pressure on water covering plant surfaces forces water into surface apertures despite air bubbles and the waxy nature of the pore surfaces. The cooling of fruits and vegetables leads to a reduction of gas pressures in the apoplast [54], particularly if the surface apertures are clogged with liquid. This pressure differential would persist until internal temperatures and gas pressures equilibrate with the external environment. Tomatoes allowed to cool while submerged in water may increase in weight due to water uptake [30,51]. Hydrocooled tomatoes increased in weight as they cooled [34] as did hydrocooled strawberries [52]. If the water contained cells of Erwinia carotovora subsp. carotovora or spores of Botrytis cinerea, water uptake correlated with a rapid development of internal lesions when the tomatoes or strawberries, respectively, were subsequently stored. When submerged in an aqueous cell suspension of E. coli at 2°C, fruit of four different apple cultivars initially at 22°C internalized the bacterium in the outer core region of the fruit during a 20-minute exposure [53]. A water-soluble dye was observed to internalize in similar treatments. However, evidence for the penetration of the skin, likely through open lenticels, appeared to be limited to injuries to the surface. Kenney et al. [55] observed E. coli cells up to 24 ^ deep in open lenticels on "Delicious" apple fruit that had been cooled in an ice bath. Bruising the surface increased the number of internalized bacteria, particularly with respect to those embedded in cracks in the surface waxes. However, washing the apples in distilled water prior to examination led to an apparent reduction in penetration to depths no greater than 6 Cooling hot water-treated mango fruits (46°C) in water (22°C) for 10 minutes led to infiltration of the fruit by a dye solution or by a suspension of Salmonella enterica [56]. The dye and bacteria primarily entered through the stem scar.

Direct injury to plant tissues may be caused by an infiltration by water, likely because the congestive water is absorbed by the cells causing them to swell. Tomato fruits that absorbed water equal to 3% or more of their original weight developed visible cracks, usually near the shoulders [30]. Studer and Kader [51] reported that a high percentage of freshly harvested tomatoes submerged for 15 to 120 minutes in water of various temperatures developed splits (breaks in the surface), whereas those stored overnight before the water treatment did not. Warming the water to above the fruit temperatures reduced but did not prevent the splits.

Hydrostatic pressure also can force water into apertures on fruits and vegetables [32]. Fruit or vegetables at the bottom of containers of submerged products would be exposed to a hydrostatic pressure on product surfaces equal to the total depth of submersion. Hydrostatic pressures would be additive to pressure differentials associated with cooling but counteract those associated with warming. However, water depth pressures would be exerted more rapidly than those associated with temperature changes. Hydrostatic pressures not only directly force water into surface apertures but also tend to squeeze submerged products and may cause air to bubble out of openings. When the hydrostatic pressure is removed, the product is likely to expand to its original volume leading to an internal pressure differential that will draw water into the product.

An abrupt impact with water can cause microbial internalization and water channels in fruits and vegetables. Water impact forces occur when field containers of freshly harvested fruits or vegetables are emptied into water or when a pile of a product is dispersed into a packinghouse flume by a heavy stream of water. Pressure washing systems in packinghouses also are likely to produce water congestion in surface apertures.

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